Aromatic polycarbonates have excellent mechanical properties such as impact strength, heat resistance, transparency and the like, and are widely used to applications such as various machine parts, optical discs and automobile parts.
A method of directly reacting bisphenols such as bisphenol A and phosgene (interfacial method) and a method of polycondensation reacting bisphenols such as bisphenol A and carbonic diesters such as diphenyl carbonate by an ester exchange reaction (melt process) are known as production methods of such aromatic polycarbonates. Above all, the melt process by an ester exchange reaction has the advantage that polycarbonates can be produced inexpensively as compared with the interfacial method.
By the way, polycondensation reaction between an aromatic dihydroxy compound and a carbonic diester by a melt process is generally conducted using a multi-stage reactor in the presence of an ester exchange catalyst (see Patent Document 1).
Further, many examples are hitherto reported on aromatic polycarbonates by a melt process. For example, there are a method in which temperature difference between a polymer temperature and a heating medium in a reactor is 100° C. or less when a limiting viscosity [η] of an aromatic polycarbonate formed is 0.2 or less, the temperature difference is 80° C. or less when [η] exceeds 0.2 and is 0.35 or less, and the temperature difference is 50° C. or less when [η] exceeds 0.35 (see Patent Document 2); a method of automatically controlling a polymer viscosity at the outlet of a final reactor by changing temperature or pressure of reactors according to programs previously installed, based on the measurement values (see Patent Document 3); and a method in which difference between outer wall surface temperature of a piping which transfers a molten polymer having high molecular weight and temperature of a molten polymer in a polymerization vessel is in a range of from −3° C. to 50° C. (see Patent Document 4).
Further, there are a method of transferring an aromatic dihydroxy compound and a diaryl carbonate having been subjected to vacuum substitution using an inert gas to a raw material dissolving and mixing tank heated to 115 to 220° C. to adjust a rate of reaction to a range of from 5 to 95% and then transferring the mixture to a post-step, followed by polymerizing (see Patent Document 5 and Patent Document 6); a method of preparing a polycarbonate prepolymer, passing a step of controlling a hydroxyl group terminal ratio of plural prepolymers, and simultaneously continuously producing plural polycarbonates having different molecular weight in a post-polymerization step (see Patent Document 7); and a production method which combines one first polymerization step and plural later polymerization steps (see Patent Document 8).
Further, there is a production method in which surface temperature of a reactor material is controlled to a temperature of 230° C. or higher, thereby suppressing crystallization of a low-order polycondensate of a polycarbonate formed in the course of a polycondensation reaction (see Patent Document 9).
A melt process generally involves an operation of setting operation conditions of a reactor to higher temperature and vacuum conditions as increasing stages to effectively remove phenol by-produced, thereby increasing the degree of polymerization of a polymer formed. Further, to avoid hue of a polymer from deteriorating, it is required to decrease thermal history received as possible and to remove phenol by-produced in a short period of time.
From the above standpoints, the present applicant reported a method of producing a high molecular weight aromatic polycarbonate having excellent hue by controlling an average residence time of a molten reactant in each reactor and stirring powder per unit volume of a molten reactant when producing an aromatic polycarbonate using a production apparatus comprising a plurality of vertical reactors and at least one horizontal reactor, connected in series (see Patent Document 10).
Patent Document 1: JP-A-05-239334
Patent Document 2: JP-A-06-065365
Patent Document 3: JP-A-06-065366
Patent Document 4: JP-A-10-330474
Patent Document 5: JP-A-2003-034719
Patent Document 6: JP-A-2003-034720
Patent Document 7: JP-A-2003-192782
Patent Document 8: JP-A-2004-026916
Patent Document 9: JP-A-2000-198839
Patent Document 10: JP-A-2001-200047